The present invention relates to an image coding apparatus for compressing and coding a video signal.
MPEG-2 is known as one of the high-efficiency video compression standards issued by Moving Picture Experts Group. In accordance with the MPEG-2 standard, each video frame (or field) is classified as an I, P or B frame. An I-frame is intra-frame coded using no reference frames. On the other hand, P- and B-frames are inter-frame predictively coded using some reference frames. Specifically, a P-frame is an abbreviation of a predictively coded frame, which is also called a xe2x80x9cpredicted framexe2x80x9d. A B-frame means a bidirectionally coded frame, which is also called a xe2x80x9cbidirectionally predicted framexe2x80x9d. Each frame is also subdivided into a plurality of units called xe2x80x9cmacroblocksxe2x80x9d, each of which is composed of 16xc3x9716 pixels. And each frame is coded by being subjected to discrete cosine transform (DCT) on a block-by-block basis, where each block is composed of 8xc3x978 pixels.
According to a technique disclosed in Japanese Laid-Open Publication No. 9-271026, when video with a relatively high coding complexity is input, the resolution of succeeding video signals is decreased to constantly control the number of binary bits generated per unit time at a predetermined number or less. This countermeasure is taken when either the product of a quantization scale (i.e., the magnitude of a unit quantization step) and the number of bits generated or an occupied buffer capacity is about to reach a certain limit. Thus, if the number of bits generated is going to exceed the predetermined number, the resolution must be decreased even if the input video signal contains a lot of high-frequency components. Accordingly, deterioration of image quality is unavoidable in such a situation.
It is therefore an object of the present invention to provide an image coding apparatus that can greatly cut down on the number of bits generated while minimizing the deterioration in image quality.
To achieve this object, the inventive apparatus adaptively changes the resolution according to the characteristics of an input video signal. In an exemplary embodiment of the present invention, a resolution for an input video signal is increased with respect to complicated input video with a relatively high coding complexity. Conversely, the resolution is decreased with respect to monotonous input video with a relatively low coding complexity. For further details, see the detailed description of the invention.